Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as medical, automobile, and other applications. High dynamic range (HDR) image sensors have been required by many of those applications. Human eyes normally possess a dynamic range of up to about 100 dB. For automobile applications, an image sensor of more than 100 dB dynamic range to deal with different driving conditions, such as driving through a dark tunnel into bright sunlight, is often required.
HDR image sensors do not always perform HDR functions properly. Common drawbacks include image degradation due to fixed pattern noise, large random noise, reduced resolution associated with charge blooming, motion artifacts, fixed sensitivity, and lower fill factor when multiple photodiodes are used, where fill factor is a ratio of a pixel's light sensitivity area to its total area.
When HDR image sensors are used to receive lights from LEDs, artifacts are common places due to the duty cycle of LED lights. LED has a tendency to emit lights out in pulses instead of in continuous and steady fashion. That has made the capture of those LED light pulses difficult due to mismatch between the emitting pulses of the LED and the receiving pulses of image sensors. For example, LED traffic lights may flicker several hundred times per second. LEDs used in head lights and brake lights in many modern cars behave very similarly.
A typical LED is normally driven by a pulse-width modulation (PWM) controller. The use of PWM has benefits such as adjustable light intensity through duty cycles, less heat buildup to extent life span, and power savings. When the LED pulse is on, it provides high lighting condition. When the LED pulse is off, it provides low lighting condition. The modulation frequency runs from a few tens to a few thousands of hertz. That's normally not noticeable to human eyes. However, rapid LED pulse makes it difficult for the conventional method, even guarded with HDR capability, to perform proper LED imaging. Since the capture rate of the sensor can easily mismatch the emission rate of the LED, for most of the time, LED light cannot be caught evenly from frame to frame. Even the double exposure technique cannot mitigate the LED flicker effectively. One of the worst case of this is automobile lights made of LEDs that need to be detected when blended with the strong reflection of the sun light.
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